Every weapon skin in CS2 belongs to exactly one collection. Case collections share a name with their case, such as the Chroma 3 Collection, and hold skins from Mil-Spec up to Covert plus a gold Rare Special Item slot. Map collections are named after maps, such as the Dust 2 Collection or the Ancient Collection, and they hold skins from Consumer Grade up to Classified or Covert with no knife slot.
Map collection skins drop from weekly care packages, from operation rewards, and as Souvenir items from Major matches. Some older collections no longer drop at all, so their supply is fixed.
Why do collections matter for trade-ups?
A trade-up contract returns a skin from the next rarity tier, drawn only from the collections of the ten inputs. If all ten inputs are from one collection, the output is one of that collection's skins in the next tier. If you mix collections, each collection's share of the ten inputs sets its share of the odds, and within a collection every possible output is equally likely.
This makes collection choice the core skill of trade-ups. A collection with one expensive output and several cheap ones offers a gamble; a collection with all outputs above the input cost offers a safer contract. Traders also check whether a collection even has skins in the target tier, since a contract cannot output a tier the collection does not contain.
How can you tell which collection a skin belongs to?
The collection name shows in the item description in your inventory, on the Steam Community Market page for the skin, and in most third-party price sites. Some collections were released alongside operations and share a theme, so the name is often a clue about when the skins came out and whether they still drop.